1. Field of the Invention
The present invention relates to a dry process for forming patterns on a substrate, wherein an unzippable polymer is employed as a screen to increase screening resolution in both lateral and thickness dimensions.
In a preferred embodiment, a suitable unzippable polymer, with or without a UV absorbing dye, is solution cast or spin coated on a substrate. The unzippable polymer is then laser ablated to form the desired pattern. A paste is then screened through the ablated polymeric pattern onto the substrate. After coating paste deposition, the sample is heated to depolymerize the unzippable polymer thereby destroying the adhesion of the unzippable polymer to the substrate; and, the unzippable polymer volatilizes off the substrate leaving behind the coating paste adhered to the substrate in the ablated pattern.
2. Description of the Prior Art
U.S. Pat. No. 4,508,754 to Stepan discloses a method for adding fine line conductive/resistive patterns to a thick film microcircuit. More particularly, the method comprises cutting a circuit pattern on a substrate; filling the circuit pattern with a conductive or resistive paste and removing the excess; adding screen printing thick film circuit elements on the substrate surface and over selected portions of the fine line pattern; and heating, thereby sintering the paste and thick film elements to the substrate.
U.S. Pat. No. 4,456,675 to Anderson, Jr., et al. relates to a dry process for forming metal patterns wherein metal is deposited on a depolymerizable polymer and selectively removed. The process comprises forming a mask of a thermally depolymerizable polymer at a temperature above 350.degree. C. on a substrate with a pattern of openings complementary to the desired metal pattern; blanket coating the substrate and the mask with a metal; heating the substrate to depolymerize the depolymerizable polymer; cooling the surface of the metal to thereby delaminate the metal coated in the areas where the thermally depolymerizable polymer is present; removing the delaminated metal where necessary, and optionally plasma ashing the depolymerized polymer, if residue thereof remains, to remove same from said substrate.
U.S. Pat. No. 4,539,222 to Anderson, Jr., et al. discloses a wet process for forming a desired metal pattern on a substrate which comprises forming a mask of a thermally depolymerizable polymer on the substrate with a pattern of openings corresponding to the desired metal pattern; blanket coating the substrate and the mask with a metal; heating to depolymerize the thermally depolymerizable polymer, and removing the thermally depolymerizable polymer and metal thereover in a mild solvent.
U.S. Pat. No. 4,842,677 to Wojnarowski, et al. relates to a dual layer photoresist system which is applicable to the formation of high resolution conductive patterns on insulative substrates with non-planar surface variations. The method comprises disposing a thin layer of ablatable photoabsorptive polymer on a substrate containing a conductive layer; directing a beam of laser energy on the ablatable polymer through a mask so as to ablate the polymer layer partly exposing the conductive layer; disposing a thick layer of substantially transparent material over the partially ablated polymer layer and partially exposed conductive layer wherein the photoabsorptivity and transparency are determined with respect to the same frequency range; and directing a beam of laser energy through the thick layer so as to ablate the polymer layer and to simultaneously remove the thick layer above the ablated polymer layer, whereby select portions of the conductive layer are exposed.
U.S. Pat. No. 4,865,873 to Cole, Jr., et al. discloses a method of electrolessly plating a metal on a substrate. The method comprises coating the substrate with at least one layer of an ablatively-removable material; irradiating at least a portion of the coated layer with a sufficient amount of laser radiation to ablatively remove the irradiated portions of the coated layer; coating the irradiated substrate with a catalyst capable of instigating the electroless deposition of a metal to be plated when contacted with an electroless plating solution; and contacting the catalyst coated substrate with an electroless plating solution to plate the metal on the irradiated portion of the substrate, and, remove the unirradiated catalyst-coated layer.
The method of the present invention obviates many of the disadvantages inherent in prior art processes. More specifically, the present invention does not employ currently used wet procedures, which include a solvent exposure, to remove organic masking layers. This prior art technique can lead to device solvent contamination and adhesion loss with polymer containing devices and structures. Rather, the present process employs a laser ablatable unzippable polymer as a masking layer, which is thermally depolymerizable and does not require solvent exposure to effect lift-off.
For instance, many prior art wet procedures employ a lift-off layer comprising a polysulfone. This lift-off layer is typically removed by soaking in hot N-methyl pyrrolidine (NMP) for about 8 to about 18 hours at elevated temperatures, i.e., 85.degree. C.-130.degree. C. The solvent dissolves the polysulfone layer and the overlaying metal film leaving behind the desired metallization pattern. Typical disadvantages inherent with this technique include long soak times in hot NMP; strong polar solvents such as NMP readily swell organic polymers such as polyimides which results in increased volumn of the organic polymers which can stress and crack the barrier layers; NMP is readily absorbed into organic polymers and are extremely difficult to remove from the device; and, polysulfone is sensitive to electron beam radiation which apparently leads to cross linking, resulting in reduced solubility and longer NMP soak times to effect lift-off.